1. Field of the Invention
The present invention relates to a superconducting magnet apparatus configured to generate a high magnetic field.
2. Description of the Related Art
When a superconducting coil is cooled to an ultra-low temperature, the electrical resistance of the superconducting coil becomes zero. By using this characteristic, the superconducting coil can increase current density without generating joule heat, and hence the superconducting coil is suitable for generating a high magnetic field. A superconducting magnet apparatus configured by using the superconducting coil having such characteristic is widely used as a high magnetic field generator in the field of research of physical properties, and the like.
Here, the superconducting coil needs to be cooled to an ultra-low temperature of about 4 K, and hence liquid helium, or the like, is used as a refrigerant.
The liquid helium is difficult to be directly handled and is not an abundant resource, and hence in recent years, the method has been widely used, in which the superconducting coil is cooled by using an ultra-low temperature refrigerator.
With the spread of the ultra-low temperature refrigerator, the practical use of, in particular, a high temperature superconductor has been rapidly advanced.
For example, a high magnetic field generator using a small refrigerator has been developed by combining a high-temperature superconducting coil with a low-temperature superconducting coil (see, for example, TEION KOUGAKU (J. Cryo. Soc. Jpn.), Vol. 41, No. 7, P 322-327).
In the superconducting magnet apparatus described above, it is sufficient, in the steady state, only to cool the amount of heat of about 1 to 2 W generated due to heat penetration and heat generation at a connecting portion, but the amount of heat, which is several times the amount of heat generated in the steady state, is generated in excitation or demagnetization due to magnetic hysteresis loss caused by magnetic field change.
Therefore, in order to further promote the practical use of the superconducting magnet apparatus, a refrigerator having a large refrigerating capacity is required to cope with the heat generation in excitation or demagnetization.
In the low temperature superconducting coil, the magnetic hysteresis loss is reduced by developing a low-loss conductor such as a multifilamentary wire, but the magnetic hysteresis loss is large in the high temperature superconducting coil.
On the other hand, the size of the high temperature superconducting coil is required to be increased in order to obtain high magnetic field, and hence there is a problem that the refrigerating capacity of a refrigerator must be significantly increased.